1. Field of the Invention
The present invention relates to a data transmission system, as well as a method for monitoring data transmission in a computed tomography (CT) device or in an X-ray device that can be used for tomography, having a data acquisition unit in which measurement data are acquired, converted into a bit stream, and communicated to a transmitter apparatus on a rotating part of the computed tomography apparatus or the X-ray device, the transmitter then transmitting the bit stream to a stationary part of the computed tomography apparatus or of the X-ray device, and having a receiver device on the stationary part that receives the bit stream from the transmitter apparatus and communicates it to an image reconstruction unit that further processes the bit stream communicated by the receiver device for image reconstruction.
2. Description of the Prior Art
In medical imaging technology, computed tomography apparatus often are used in which a very large amount of measurement data is simultaneously acquired, communicated to an image reconstruction unit, and further processed in order to reconstruct the desired images. The data transmission system required for this must be capable of high-speed transmission, due to the large amount of measurement data that occurs per time unit, and must also ensure a maximally error-free transmission between the rotating part of the computed tomography apparatus (the gantry) and the stationary part. Similar requirements exist in the case of C-arm X-ray devices. Such X-ray devices, or other X-ray devices capable of tomography, are subsumed under the term “computed tomography device” below.
Various technologies are known for the data transmission between the rotating part and the stationary part.
The known technologies can be divided into transmission technologies using capacitive coupling and those using optical coupling. In transmission using capacitive coupling, the signals are transmitted from a transmitter fastened to the rotating part to an antenna situated on the stationary part. Thus, German OS 100 07 601 specifies an apparatus for data transmission in which a waveguide is used as a transmitter. For the data transmission, the data are modulated onto a carrier signal and are coupled into the waveguide. An antenna situated in a particular geometrical fashion relative to the waveguide receives the carrier signal in contactless fashion, so that after demodulation of the carrier signal the data are available at the stationary part. In the application depicted in this publication, the waveguide is fastened along the periphery of the C-arm of a C-arm X-ray device, and the antenna is fastened to the mount of this C-arm.
U.S. Pat. No. 5,140,696 specifies an apparatus for signal transmission between elements that are moved relative to one another, in particular in a computed tomography apparatus, in which as a transmitter a circular strip conductor is situated on the periphery of the gantry, and as a receiver a short segment of a strip conductor is provided on the stationary part in the immediate vicinity of the transmission line. The data transmission takes place in the same manner as in the reference cited above.
In signal transmission using optical coupling, the transmission of the data takes place via an optical interface. Thus, U.S. Pat. No. 4,259,584 specifies an apparatus for signal transmission, in particular for a computed tomography apparatus, in which on the stationary part there is fashioned a ring that runs around the rotational center and is made of an optical waveguide at the output point of which there is situated a demodulator. On the rotating part, opposite the optical waveguide a light source is fastened, the intensity of which is modulated with the data that are to be transmitted. The modulated light signals are constantly coupled into the optical waveguide ring during the relative movement due to this geometrical arrangement, and are received by the demodulator, which extracts the data by demodulation.
U.S. Pat. No. 5,535,033 discloses a signal transmission apparatus in which a ring made of an optically conductive material is fastened on the rotating part of a computed tomography apparatus as a part of a transmission apparatus that also radiates the coupled-in light perpendicular to its longitudinal axis. The data to be transmitted are coupled into this ring by modulation of a light source, and are received at the stationary part via an optoelectrical detector. Due to the annular construction of the transmitter apparatus, here as well reception of the data by the receiver is possible during almost every phase of rotation.
Independent of the data transmission technology used, a computed tomography device has a data acquisition unit that converts the measurement data obtained from the multiplicity of detector channels into a bit stream, which generally is serial, and communicates this bit stream to a transmitter device on the gantry. The transmitter device transmits the serial bit stream to a receiver device on the stationary part of the computed tomography device, which in turn forwards this bit stream to the image reconstruction unit, in which the bit stream generally is first again demultiplexed and subsequently further processed for image reconstruction. This data connection between the data acquisition unit and the image reconstruction unit is relatively complex due to the numerous components involved, so that transmission errors that may occur can be diagnosed only with difficulty. This is true both for the design and integration phase and for the manufacturing phase of the system; in each of these phases it is difficult to test the quality of the data connection and to identify determinate points in the data transmission chain. Additionally, when data transmission errors occur in the clinical environment it is very difficult and time-intensive, and thus expensive, for service personnel to discover the faulty components in the data chain. In computed tomography devices, the testing of data transmission is made more difficult by the fact that during the operation of the computed tomography device the gantry rotates continuously, so that it is almost impossible to couple test devices to the data acquisition unit as a source of data.
In order to test the quality of data connections, from communication technology the acquisition and evaluation of the bit error rate (BER) is known, which indicates the number of bits transmitted with errors in relation to all the bits communicated in a predetermined interval. For testing data transmission systems, special measurement devices are available for determination of the bit error rate; these devices are known as BERT (Bit Error Rate Tester) systems. These test systems contain a bit pattern generator that sends a predefined bit sequence via the data transmission system, and an error analysis unit that analyzes the transmitted bit sequence. A reference clock pulse is transmitted via a direct connection between the bit pattern generator and the error analysis unit in order to correctly read out the obtained bit sequence. FIG. 3 shows, as an example, such a test system 22, with a bit pattern generator 23 and an error analysis unit 24, for testing a data transmission system 26. The error analysis unit 24 enables the acquisition of the number of errors during the data transmission, the classification of the errors, and the determination of the position of the errored bit within the data stream, so that from these data conclusions can be made concerning the cause of the errors. Examples of such an error analysis can be found in the following publications: G. M. Foster and T. Waschura, “Beyond Bit Error Ratio B Gain New Insight from Studying Error Distributions,” Agilent Technologies Technical Paper, Literature No. 5988-8037EN, Sep. 26, 2002, 8 2002 Agilent Technologies, http://literature.agilent.com/litweb/pdf/5988-8037EN.pdf, and “An Introduction to Error Location Analysis B Are All Your Errors Truly Random?,” Agilent Technologies Application Note 1550-2, Literature No. 5980-0648E, April 2000.
Such a known test system cannot be used in computed tomography devices, because the data acquisition unit as a data source and the image reconstruction unit as a data receiver are continuously rotating relative to one another. Most available test systems contain the bit pattern generator and the error analysis unit in the same housing, so that a physical separation of them is not possible. Even if the bit pattern generator were provided separately, it would be practically impossible to fasten it to the rotating gantry, due to the small space available and the resulting disturbance to the mechanical equilibrium. In addition, in a computed tomography device there is only one high-speed connection between the rotating part and the stationary part, so that no separate connection is available for the communication of the reference clock pulse. A further problem is presented by the complex transmission chain in a computed tomography device, in which first a parallel data stream is converted into a serial bit stream, the data are coded in order to integrate the clock signal into the data stream, the serial bit stream is transmitted between a continuously rotating part and the stationary part, the contained clock signal is extracted in order to sample the data using the extracted clock signal, and finally the serial data stream is converted back into parallel words. For the testing of such a transmission chain, parallel test systems are necessary, requiring a very expensive and complex interface to the test system.
For this reason, heretofore service personnel in the clinical environment have approached the problem of finding the cause of data transmission errors in computed tomography devices simply by successively exchanging individual components. This trial and error technique is very time-intensive and expensive, because all parts that can be exchanged must be kept available. In addition, this technique is useless in the case of causes of error that occur from an outside source, for example errors that occur due to interference between external sources of disturbance and the transmission between the transmitter apparatus and the receiver apparatus of the computed tomography device.